1. Field of the Invention
The present invention relates to the manufacture of reticles used in fabricating semiconductor devices. More particularly, the present invention relates to determining the accuracy of the reticle formation and a method for determining distortions in the patterning of the reticle such that the method enables a user to decide whether or not the reticle should be used to form patterns on films deposited over semiconductor wafers.
2. Description of the Related Art
Today""s semiconductor devices are continually being pushed to meet stricter demands. As devices using this technology inundate the marketplace, consumers place higher demands on the devices. These demands include smaller, more compact devices with greater functionality.
Semiconductor devices employ various circuitry in a chip to perform user specified functions. As is well known, the circuitry consists of various metallization lines, dielectric layers and other components interconnected throughout the entire chip. The metallization lines and dielectric layers are formed by first depositing a metal layer or a dielectric layer onto a semiconductor wafer. The metallization lines and dielectric layers are deposited on the semiconductor wafer by spin coating, chemical vapor deposition, deposition and other standard techniques of applying films onto wafers. After deposition, the film must be patterned to form a metallization layer or other component on the semiconductor wafer.
Now making reference to FIG. 1, a photoresist layer 104 is applied by spin coating photoresist over a deposited layer 101. Once the photoresist layer 104 is spin coated onto a wafer 102, the wafer 102 is patterned. The wafer 102 is placed into a stepper that contains a reticle 100 which has a proper pattern 100a for the deposited layer 101. The stepper machine transfers the image of the reticle 100 onto the semiconductor wafer 102 by passing a light source 103 through the reticle 100. The reticle 100 acts as a filter and only allows a certain pattern of light from the light source 103 to pass through and onto the photoresist layer 104 of the wafer 102. The pattern of the light passing through the reticle 100 is the pattern for a feature to be formed on the deposited layer 101 of the semiconductor wafer 102. The light 103 passing through the reticle 100 and onto the photoresist layer 104 will react with the photoresist layer 104. The reaction will affect the solubility of the exposed portions of the photoresist layer 104 when the photoresist layer 104 is immersed in a solvent. For example, if the photoresist is positive, it will become more soluble as it is exposed to the light. Therefore, photoresist 104a will become soluble when it is subjected to an immersion process (not shown), leaving the pattern defined by photoresist 104b. If the photoresist is negative, the photoresist will become more insoluble as it is exposed to the light. For example, the photoresist 104b will dissolve and the photoresist 104a will remain after the photoresist 104 subjected to an immersion process (not shown).
The pattern for the reticle which is used to pattern the film is first designed in an integrated computer design (IC) using a computer aided design program. After the design is made, the features of the design which are to be formed on the wafer must be transferred to the reticle. For example, what will define a metallization line in the digital IC design will be transferred to the reticle in order for it to be imaged onto the photoresist 104 of the semiconductor wafer. The reticle is a glass plate which will be placed into the stepper. Chromium is deposited on top of the glass plate by any standard technique. A photoresist layer is then spin coated over the chromium layer. Once the photoresist is deposited over the chromium layer, the photoresist is patterned with either a laser tool or using an electron beam direct write exposure technique. After the image is formed on the reticle, the reticle is ready to pattern films on semiconductor wafers.
However, in most cases, the image that appears on the reticle will not be the same image that is in the computer digital IC layout. For example, a rounding effect may occur at sharp edges, such as those edges used to define a square feature on the reticle. The square feature will have rounded edges approximating parabolas instead of edges approximating the corners of the square. This effect occurs due to well known proximity effects and because features of the digital IC design are being designed at such a small scale that it is difficult to reproduce the same digital image on the reticle.
In order to avoid these problems, designers typically employ serifs at the edges of a feature in the digital IC design which is transferred to the reticle. For example, serifs would be placed in the corners of the aforementioned square to compensate for the rounding effect which takes place in the corners of the square. The serif increases the amount of area in each of the corners to compensate for anticipated losses. Thus, the area created by the serifs will compensate for the loss of area due to the rounding of the edges within a square feature not containing the serifs.
Nonetheless, the use of serifs in the prior art creates problems because many changes may be required to get proper image formation on a wafer. With the current available methods, a user is unable to determine how all the changes (e.g., proximity effects) will change the original digital IC design after it is transposed onto a reticle and finally onto the film of a semiconductor wafer. Furthermore, the prior art checks on reticles only involved checking proximity effects due to close line separations, while no checks can be made to determine problems with the overall image, such as corners. As a consequence, the image formed on the reticle may not function as originally intended because the image has experienced too many distorting changes which are not appreciated until the reticle transfers the patterns to photoresist.
Typically, even before a reticle will be used to form patterns on the film, a determination must be made as to the quality of the reticle (e.g., how planar is the glass, imperfections on the surface of the reticle and concavities which render the reticle impractical for use). This determination is usually done by placing test patterns on the reticle and then ascertaining how closely the design on the reticle conforms to the design in the digital IC layout.
However, as mentioned above, the quality of the reticle can only be tested by placing the reticle with the test pattern into the stepper and forming patterns on a film of the wafer itself. This greatly slows down the process time since this requires a user to wait until the photoresist develops before one may determine the accuracy of the photoresist image. Also, a user is precluded from making a series of reticles without worrying about whether or not the reticles form the desired images on the films. Instead, after one reticle is made, a user must use-the developed photoresist layer to determine the reticle""s accuracy. Furthermore, as is well known in the art, there is a general push to generate reticles more quickly in order to facilitate the manufacture of semiconductor wafers.
As a result, the current methods of checking reticles is time consuming and expensive. A user must actually place the reticle into the stepper and print the reticle image with photolithography before a determination can be made as to the quality of the reticle. Also, the time of using the stepper, developing the photoresist and using the wafer increase the cost of ascertaining the quality of a reticle.
In view of the foregoing, there is a need for a method of determining the accuracy of a particular reticle which avoids the problems of the prior art. This new method should facilitate checking reticles in a time efficient and cost efficient manner.
Broadly speaking, the present invention fills these needs by providing a method for inspecting features on a reticle. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, a method for inspecting features on a reticle is disclosed. A layout design of a test feature is first provided and then the layout design is transferred to a reticle. After the layout design is transferred to the reticle, an image of the transferred layout design is captured. The captured image of the transferred layout design is compared against the layout design of the test feature to ascertain deviations between the captured image and the layout design once the image is captured.
In another embodiment, a method of ascertaining a degree of distortion of features on a reticle is disclosed. A test feature is provided and then transferred to a reticle to create a transferred test feature. After the test feature is transferred, the transferred test feature is compared to the test feature to ascertain the degree of distortion. The transferred test feature is also compared with the test feature to determine whether modifications of the test feature are necessary to compensate for the degree of distortion of the transferred test feature.
In yet another embodiment, a computer readable media having program instructions for carrying out a method of ascertaining a degree of distortion of features of a reticle is disclosed. Programming instructions provide test features and transfer the test feature to a reticle to create a transferred test feature. Programming instructions also compare the transferred test feature to the test feature to ascertain the degree of distortion and whether modifications of the test feature are necessary to compensate for the degree of distortion of the transferred test feature.
The many advantages of the present invention should be recognized. The present invention gives a user a simple way of determining the acceptability of a reticle by measuring features transferred onto a reticle against the digital version of the features. This method avoids the prior art problems of having to develop photoresist on a semiconductor wafer in order to ascertain the accuracy of a reticle. As such, this method greatly reduces costs and time in determining the acceptability of reticles.